The present invention relates generally to magnetic resonance imaging (MRI), and more particularly, to a system and method of enhanced thermal management of a gradient coil assembly in an MRI system.
Generally, MRI is a well-known imaging technique. A conventional MRI system establishes a homogenous magnetic field, for example, along an axis of a person's body that is to undergo MRI. This homogeneous magnetic field conditions the interior of the person's body for imaging by aligning the nuclear spins of nuclei (in atoms and molecules forming the body tissue) along the axis of the magnetic field. If the orientation of the nuclear spin is perturbed out of alignment with the magnetic field, the nuclei attempt to realign their nuclear spins with an axis of the magnetic field. Perturbation of the orientation of nuclear spins may be caused by application of radio frequency (RF) pulses. During the realignment process, the nuclei process about the axis of the magnetic field and emit electromagnetic signals that may be detected by one or more gradient coils placed on or about the person.
As well known in the MRI industry, high power MRI systems consume large amounts of electrical power. In particular, the gradient and RF coils consume excessive amounts of power and thus these coils generate significant heat typically on the order of tens of kilowatts. As one would expect, excessive heat can cause system components to deteriorate or fail prematurely and hence adversely affects reliability. Moreover, rise in temperatures causes a reduction in coil currents and low coil currents affects signal generation, resulting in poor image resolution. In addition, heat can be an annoyance to a patient during the imaging process and, if excessive, could injure a patient. For this reason there are regulations that stipulate the maximum temperature of a patient support table that effectively limit the amount of power that can be used in any MRI system.
One way to minimize heat is to provide a hermetically sealed liquid cooling system with cooling conduits adjacent the gradient coils. According to solutions of this ilk, during field generation and data acquisition liquid coolant (e.g., water) is pumped through the system to cool the coils. Liquid cooling offers effective cooling of the system and the components and therefore can have higher currents and cooling efficiency as compared to other cooling systems like an air-cooled system.
Current cooling circuits based on liquid cooling method in MRI gradient coils are of serpentine scheme with either copper tubes or lengthy plastic tubes. One of the problems of the long cooling tubes is that it introduces large pressure drops. In addition, there is another limitation on the number of tubes that can be deployed in the radial direction at a given radius around the perimeter of the coil due to practical limitations in bending the tube. Higher image quality MRI machines require higher power density and higher power density increases heat generation. The heat must be effectively removed. A typical MRI system generates heat loads of 10–18 kW depending on the field strength. For example, a high-end 7 Tesla (7T) system generates about 17 kW of heat or more depending on the pulsing mode.
Moreover, serpentine cooling passages traverse through the gradient coils several times before the heat can be removed from the coolant. In addition, the serpentine cooling tubes as found commonly are mostly made from copper or aluminum tubes or similar metals due to high thermal conductivity, low cost and availability reasons. Due to likeliness of the eddy loops in presence of a magnetic field, the metallic material may give rise to continuous eddy loops in the cooling tubes. Therefore, it is necessary to have an electrical break in the cooling system. Due to low coverage area of the serpentine cooling, constriction resistance is higher and it is more prone to have temperature non-uniformities and local hot spots. In addition, there is high vibration associated with the operation of a higher field strength MRI system and from a structural point of view, a change from the serpentine cooling scheme is needed. There is need of an enhanced thermal management of the MRI system. This new system will provide both lower spreading resistance and higher cooling capacity.
It would therefore be desirable to design an enhanced method and system to maintain gradient coil temperature within a specified range regardless of the selected excitation applied, thereby enabling higher power applications for faster imaging with improved image quality and longer scan times.